1. Field of the Invention
The present invention relates to the vaporization of a liquid by exchanging heat with a second fluid by means of a heat exchanger of the vertical plate type. It applies in particular to air distillation installations.
2. Related Art
In air distillation installations of the double column type, the liquid oxygen that is in the vessel of the low-pressure column is vaporized by exchanging heat with the gaseous nitrogen tapped from the head of the medium-pressure column. For a given operating pressure of the low-pressure column, the temperature difference between the oxygen and the nitrogen made necessary by the structure of the heat exchanger imposes the operating pressure of the medium-pressure column. It is therefore desirable that this temperature difference be as small as possible in order to minimize the expenditure associated with compressing the air to be treated injected into the medium-pressure column.
The technology currently used for these phase-change exchangers is that of aluminum exchangers with brazed plates and fins which make it possible to obtain very compact members providing a large exchange surface area. These exchangers consist of plates between which waves or fins are inserted thus forming a stack of vaporization “passageways” and of condensation “passageways”. There are various types of waves such as straight, perforated or partially offset (“serrated”) waves.
In the case of vaporizers operating in descending-film vaporization mode, a portion of the apparatus is dedicated to the distribution of the liquid in the vaporization passageways and between the channels of the exchange wave.
This distribution specific to each vaporizer is carried out conventionally according to the principle described in FR-A-2547898: the vaporization passageways are supplied through the top of the condensation passageways. The oxygen then passes through an array of holes which ensure its primary distribution into the vaporization passageways. It then flows through a band of waves with a horizontal generatrix which ensures a finer distribution called secondary distribution (dividing of the liquid between channels).
The liquid oxygen that is vaporized contains impurities in the form of solutes. The main impurities are nitrous oxide (N2O), carbon dioxide (CO2), hydrocarbons (C2, C3, etc.). Depending on the operating conditions, these impurities may be deposited in the vaporization passageways (either in solid form or in liquid form). It is important to industrially control the formation of these solid or liquid deposits in order to prevent any risk of explosion.
One of the important parameters in the formation of deposits is the liquid flow rate per channel (or expressed per meter of perimeter to be wetted). Specifically, when the liquid flow rate per channel is insufficient to wet the wall, there is formation of deposits by dry vaporization.
In this type of vaporizer (film vaporizer), the distribution of the liquid oxygen plays an essential role in its operation (performance and safety). It is therefore necessary to ensure, in all circumstances, a good liquid distribution inside each channel. For this, the liquid distribution must be sufficiently uniform between channels. A non-uniform liquid distribution may cause bad wetting of the waves, notably in the bottom portion of the exchanger, and consequently the formation of deposits by dry vaporization. The difficulty is in ensuring an equivalent liquid flow rate in all the channels considering the number of channels per passageway and per body (550 channels/passageway, 55 000 channels/body).
The quality of this liquid distribution depends on a good design and dimensioning of the distributor.
The so-called secondary distribution (dividing of the liquid between channels) uses a wave band with a horizontal generatrix and with partial offset.
The positioning of this wave inside each vaporization passageway has two drawbacks:                The existence on either side of the passageway of two orifices (free spaces) 31 due to the profile of the lateral bars used (FIG. 1A).        Possibility of clearance 31 at the junction between two wave mats. Specifically, several wave bands can be placed side by side in order to cover the whole of the width of the passageway (FIG. 2).        
These drawbacks mean that there are preferential passageways for the liquid in this band with horizontal generatrix (“hardway”), causing an oversupply of liquid to the channels situated just beneath but, more importantly, an undersupply of liquid to the channels on the periphery of the latter.
FIG. 1A shows a portion of the exchanger according to the prior art seen from above. The bar 7 has a U-shaped section piece consisting of two layers the outer one of which has a rectangular cross section while the inner one has a sinusoidal edge 20. In figure al, the same portion of the exchanger is seen from the front.
The wave with horizontal generatrix 1 is between two plates 35 defining an exchanger passageway that is closed off by a bar 7.
The clearances 31 between the waves and the bars are also indicated in FIG. 2 for an exchanger according to the prior art.